Our previous patent describes a method for the detection of a nitrogenous explosive material within an object, comprising:
(i) placing on one side of the object a source for 9.17 MeV .gamma.-rays adapted to produce the desired photon flux;
(ii) placing on the opposite side of the object a .gamma.-ray detector or an array of detectors with a nitrogen rich detection medium;
(iii) scanning the object with a .gamma.-ray beam from said source;
(iv) reading from said .gamma.-ray detector or array of detectors the total and the non-resonant attenuations of the incident photon flux; and
(v) deriving from said attenuations the net resonant attenuation and the spatial distribution thereof.
The method of our previous patent was based on the discovery made in accordance therewith that nitrogen alone manifests a resonant attenuation upon irradiation with .gamma.-rays having the energy of 9.17 MeV. In practice, the resonant component of attenuation in nitrogen, if present, is superimposed on the non-resonant attenuation which .gamma.-rays undergo in all materials. Experimentally, the two quantities directly determined are the total attenuation (resonant and non-resonant) and the non-resonant attenuation. The net resonant attenuation is then extracted from these two quantities and is indicative of the amount of nitrogen traversed by the .gamma.-rays.
As also mentioned in our previous patent, a typical source of 9.17 MeV photons is .sup.13 C which captures 1.75 MeV protons. This manner of producing the desired 9.17 MeV photons can be described by the following equation: EQU p+.sup.13 C.fwdarw..sup.14 N.sup.+ .fwdarw..sup.14 N+.gamma.(9.17 MeV)
For any specific application, this dictates the necessity for an on-site ion accelerator capable of delivering an intense proton beam of well-defined energy and optics to a .sup.13 C-containing target. For example, in the aviation-security/explosives-in-baggage detection scenario, the operational requirement of an inspection time of 6 sec. per bag implies proton beam currents of a few milliamps.
Clearly, the target design must take this factor into account as well as other requirements related to the fundamental features of the gamma attenuation process and the data normalization procedure. The total attenuation is determined by counting the 9.17 MeV resonant component of the photon flux. The non-resonant component is measured by counting photons with energies outside the resonant energy range. The net resonant attenuation is extracted from these two quantities by normalizing the former to the latter and is indicative of the quantity of nitrogen traversed by the .gamma.-rays. In principle, if the normalization is good, the net resonant attenuation will be zero (within counting statistics) for any quantity of all atomic constituents except nitrogen. The two-dimensional radiographic image of net resonant attenuations of the transmission through an inspected object is denoted a "nitrogram". The implications of these methodological features for the targets will be discussed in the following sections.
The present invention is based on some further investigation regarding the desired properties of the high energy photon source for the performance of the method of our previous patent and has for its object the selection of particularly suitable sources therefor.